Thanks, Annie, that helps a lot. So when they talk about the "half-life," they only mean that typically, or on the average, it's at half-strength after three or four hours--but even that varies a lot by patient. And even if it's at half-strength after four hours for a particular patient, it doesn't mean that the strength decreases at a steady rate, before or after. I'm catching on! Thanks for all your help,

Abby